You can use a black theme. Look at gnome-look.org
You can also reverse engineer them to look how they make the theme blank and apply it to adwaita. This is not complicated as themes are mostly CSS.
Edit to add that you can chat with people from open deskttop (gnome look) to ask your question: https://chat.opendesktop.org/#/welcome
It is good you have solved you initial issue. However, as you say, your rules are too permissive. You should not publish ports from containers to the host. Your container ports should only be accessible over reverse-proxy network. Said otherwise :3000 should not resolve to anything.
This can be simply acheive by not publishing any port on your service containers.
Here is an example of my VPS:
Exposed ports:
Redacted list of containers: